A conjugated diene rubber polymer having excellent rubber properties is widely used as an impact resistance reinforcing member of various thermoplastic resins such as an acrylonitrile-butadiene-styrene (ABS) resin and a methacrylate-butadiene-styrene (MBS) resin. Particularly, the ABS resin has good properties including impact resistance, chemical resistance, forming processability, etc., and is widely used as a material in an office machine, an electrical and electronic component, an interior material of an automobile, etc. However, the ABS resin containing a large amount of conjugated diene-based double bonds in a rubber component may be easily aged by external energy such as light or heat.
To solve the above-described limitations, a resin using an acryl rubber and a resin using an ethylene propylene diene rubber have been provided instead of a butadiene rubber, however have markedly inferior impact resistance when compared with the ABS resin. In addition, when large-sized particles are formed to improve the impact resistance, the gloss and coloring properties of a final product may be deteriorated, thereby decreasing the value of the final product.
Meanwhile, researches on improving the weather resistance of the ABS resin by adding an additive have been conducted. However, the addition of the additive or the change of a combination may increase production cost, and the commercial value of a formed product may decrease because of the surface impurities of the formed product due to the additive during processing.
Further, a method of preparing a composite rubber copolymer through mixing diene-based copolymer latex and alkyl acrylate copolymer latex, and performing an acidic enlarging process has been conducted. However, when the amount of the diene-based copolymer latex is increased to improve impact resistance, the fusion of latex during performing the acidic enlarging process may not be generated due to the difference of the glass transition temperature with the alkyl acrylate copolymer latex. Thus, the enlargement of particles may not be attained and no effect may be obtainable. In addition, the preparation of the composite rubber copolymer may be difficult due to the generation of an excessive amount of latex agglomerate.
Based on the above-described background, the present inventors have conducted researches on a method for improving weather resistance while maintaining good impact resistance and coloring properties of a diene-based rubber component, and have prepared rubber polymer latex with a multi-layer core-shell structure comprising a diene-based rubber core, a bridge shell including a vinyl-based monomer wrapping the diene-based rubber core, and an outermost shell including an acryl-based monomer wrapping the bridge shell. Then, the inventors prepared a thermoplastic resin including the rubber polymer latex with a multi-layer core-shell structure by using an ABS graft copolymer, and secured that the thermoplastic resin has good impact strength, good coloring properties and markedly improved weather resistance, thereby completing the present invention.